Powder coming out from marble

Hi, I have laid some marble tiles in my living room and this marble is from Egypt. After three months, I notice some fine powder coming out on the surface of the marble and this continues even after I have removed them with a dry cloth and a wet cloth. I need a solution to stop this. Anyone has this problem before ? Thank you

Powder coming out from marble

Powder coming out from marble

Hi, Thanks for the replies. The marble has been sealed earlier before the installation. In addition, the floor substrate has a coat of waterproofing solution, an international brand manufactured in USA. I thought initially that it is efflorescence too, however the powder also appears on the surface in the middle of the tile which measures 2 feet x 2 feet and efflorescence normally appears in the grout lines. There is no fixed interval of time for this to happen, sometimes nothing comes out for a week and then they appear every day for a short period of time. As for the cleaning, I use only clear plain water. Thanks again